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Conditional Expressions.

A conditional expression is used with the [[ compound command to test attributes of files and to compare strings. Field splitting and file name generation are not performed on the words between [[ and ]]. Each expression can be constructed from one or more of the following unary or binary expressions:
string
True, if string is not null.
-a file
Same as -e below. This is obsolete.
-b file
True, if file exists and is a block special file.
-c file
True, if file exists and is a character special file.
-d file
True, if file exists and is a directory.
-e file
True, if file exists.
-f file
True, if file exists and is an ordinary file.
-g file
True, if file exists and it has its setgid bit set.
-k file
True, if file exists and it has its sticky bit set.
-n string
True, if length of string is non-zero.
-o option
True, if option named option is on.
-p file
True, if file exists and is a fifo special file or a pipe.
-r file
True, if file exists and is readable by current process.
-s file
True, if file exists and has size greater than zero.
-t fildes
True, if file descriptor number fildes is open and associated with a terminal device.
-u file
True, if file exists and it has its setuid bit set.
-w file
True, if file exists and is writable by current process.
-x file
True, if file exists and is executable by current process. If file exists and is a directory, then true if the current process has permission to search in the directory.
-z string
True, if length of string is zero.
-L file
True, if file exists and is a symbolic link.
-O file
True, if file exists and is owned by the effective user id of this process.
-G file
True, if file exists and its group matches the effective group id of this process.
-S file
True, if file exists and is a socket.
file1 -nt file2
True, if file1 exists and file2 does not, or file1 is newer than file2.
file1 -ot file2
True, if file2 exists and file1 does not, or file1 is older than file2.
file1 -ef file2
True, if file1 and file2 exist and refer to the same file.
string == pattern
True, if string matches pattern. Any part of pattern can be quoted to cause it to be matched as a string.
string = pattern
Same as == above, but is obsolete.
string != pattern
True, if string does not match pattern.
string1 < string2
True, if string1 comes before string2 based on ASCII value of their characters.
string1 > string2
True, if string1 comes after string2 based on ASCII value of their characters.
The following obsolete arithmetic comparisons are also permitted:

exp1 -eq exp2
True, if exp1 is equal to exp2.
exp1 -ne exp2
True, if exp1 is not equal to exp2.
exp1 -lt exp2
True, if exp1 is less than exp2.
exp1 -gt exp2
True, if exp1 is greater than exp2.
exp1 -le exp2
True, if exp1 is less than or equal to exp2.
exp1 -ge exp2
True, if exp1 is greater than or equal to exp2.
In each of the above expressions, if file is of the form /dev/fd/n, where n is an integer, then the test is applied to the open file whose descriptor number is n.

A compound expression can be constructed from these primitives by using any of the following, listed in decreasing order of precedence.

(expression)
True, if expression is true. Used to group expressions.
! expression
True if expression is false.
expression1 && expression2
True, if expression1 and expression2 are both true.
expression1 || expression2
True, if either expression1 or expression2 is true.